use dynamic dispatch for visitors

[srijs]

Following up from the discussion in #40, this changes the visitor pattern to use dynamic dispatch. In my case, this reduces the text section size attributed to prost-reflect by ~6.2% from 311.8KiB to 292.4KiB.

In parallel, I'm also working to create a simple benchmark for testing the impact of changing FieldDescriptorLike, which should result in a slightly larger amount of size reduction if things go well.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.03 🎉

Comparison is base (27ca4f1) 75.34% compared to head (f061a1f) 75.38%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #41      +/-   ##
==========================================
+ Coverage   75.34%   75.38%   +0.03%     
==========================================
  Files          31       31              
  Lines        5253     5253              
==========================================
+ Hits         3958     3960       +2     
+ Misses       1295     1293       -2     

Impacted Files	Coverage Δ
prost-reflect/src/descriptor/build/visit.rs	100.00% <100.00%> (ø)

... and 2 files with indirect coverage changes

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[andrewhickman]

Thanks! In case you hadn't spotted it, there's a small benchmark for decoding at https://github.com/andrewhickman/prost-reflect/blob/27ca4f10618cd69e0f408b4ae7db808ea1374f49/prost-reflect-tests/benches/decode.rs

[andrewhickman]

Incidentally, the google protobuf well-known type descriptors are currently included as a 113KB binary blob. I don't know if that would appear in the text section, but that's probably a decent chunk of the binary size.

It would be possible to put that behind a feature flag, although without it we can't properly handle uninterpreted options.

[srijs]

Ah neat, hadn't seen the benchmark! Looks like a great starting point to do some experimentation with FieldDescriptorLike, thanks.

Incidentally, the google protobuf well-known type descriptors are currently included as a 113KB binary blob.

Oh yes, that's a good point. Feature flagging it wouldn't help me because I need the WKT definitions, although I wonder why the protoset is so large for what feels like a handful of relatively simple types?